Vehicle electrification: new challenges and opportunities for smart grids

(1)

energies

Article

Vehicle Electrification: New Challenges and

Opportunities for Smart Grids

Vitor Monteiro1,* , Jose A. Afonso2, Joao C. Ferreira3 and Joao L. Afonso1 1 _{Centro ALGORITMI, University of Minho, 4800-058 Guimarães, Portugal; jla@dei.uminho.pt}

2 _{CMEMS-UMinho Center, University of Minho, 4800-058 Guimarães, Portugal; jose.afonso@dei.uminho.pt} 3 _{ISTAR-IUL, Instituto Universitário de Lisboa (ISCTE-IUL), 1649-026 Lisboa, Portugal;}

joao.carlos.ferreira@iscte-iul.pt

* Correspondence: vmonteiro@dei.uminho.pt; Tel.: +351-253-510-392

Received: 3 December 2018; Accepted: 24 December 2018; Published: 29 December 2018  Abstract: Nowadays, concerns about climate change have contributed significantly to changing the paradigm in the urban transportation sector towards vehicle electrification, where purely electric or hybrid vehicles are increasingly a new reality, supported by all major automotive brands. Nevertheless, new challenges are imposed on the current electrical power grids in terms of a synergistic, progressive, dynamic and stable integration of electric mobility. Besides the traditional unidirectional charging, more and more, the adoption of a bidirectional interconnection is expected to be a reality. In addition, whenever the vehicle is plugged-in, the on-board power electronics can also be used for other purposes, such as in the event of a power failure, regardless if the vehicle is in charging mode or not. Other new opportunities, from the electrical grid point of view, are even more relevant in the context of off-board power electronics systems, which can be enhanced with new features as, for example, compensation of power quality problems or interface with renewable energy sources. In this sense, this paper aims to present, in a comprehensive way, the new challenges and opportunities that smart grids are facing, including the new technologies in the vehicle electrification, towards a sustainable future. A theoretical analysis is also presented and supported by experimental validation based on developed laboratory prototypes.

Keywords: vehicle electrification; smart grids; renewable energy sources; energy storage systems; power quality; bidirectional; power electronics.

1. Introduction

Nowadays, modern societies are facing the well-known problems of environmental air pollution, forcing the adoption of new strategies for mitigating greenhouse gas emissions [1,2]. Some of the actions for alleviating such emissions are mainly offered by emerging smart grids, and are sustained by: (a) Renewable energy sources (RES), on small- and large-scale; (b) energy storage systems (ESS), as a support of RES adoption; and (c) vehicle electrification encompassing advanced functionalities [3–7]. This is even more evident considering that the technologies in the field of industrial and power electronics have evolved in recent years, contributing towards a profound and motivating change of paradigm [8,9]. As a positive consequence, new electronics applications encompassing communication technologies, supported by the Internet of Thing (IoT) concept, will transform the electrical power grid into a dynamic, autonomous, secure and flexible infrastructure [10–13].

Concerning RES, in recent decades, the production of electricity from this type of source (mainly supported by wind and solar) has grown significantly as a contribution for optimizing the energy management in macro- and micro-scenarios. In this perspective, the operation and optimization aspects regarding the introduction of RES in microgrids is envisaged in [14], whereas an ample perspective of

(2)

Energies 2019, 12, 118 2 of 20

the RES contribution for disseminating the new paradigm of smart grids is presented in [15]. In order to optimize the power generation from RES, especially considering the intermittency associated with their production, it will also be fundamental, in the near future, to combine the inclusion of flexible ESS, allowing the establishiment of an efficient harmonization between power production, storage, and consumption. The present status and the perspectives for the inclusion of RES with intermittent and unpredictable production is presented in [16], the balancing strategy for power usage from RES, regarding the user demand, is presented in [17], and a review about the role of ESS for mitigating the inconsistency of energy production from RES is offered in [18].

Alongside with RES and ESS, the large-scale adoption of vehicle electrification, principally the electric vehicle (EV), will also be vital for smart grids and smart homes dissemination, as well as for reducing energy costs and greenhouse gas emissions [19,20]. A synergistic use of RES with the charging infrastructure of EVS charging toward opportunities related to the RES and EVs power optimization is offered in [21]. A complete survey concerning the electrification of transportation contextualized in smart grids is present in [22]. The collaboration of EVs and RES toward cost and emission reductions is introduced in [23]. The particular case of the power coordination between EVs and RES in a smart home level is presented in [24]. Concerning this scenario, several perspectives can be adopted. For example, smart charging approaches for EVs conceived to maximize the usage of energy from RES are introduced in [25]. Designed to enhance the grid performance, a scheduling strategy considering the uncertainties from RES and EVs is proposed in [26]. A solar docking charging station for EVs is described in [27]. The impact of EVs and solar photovoltaic panels (PV) prospecting the future enegy generation portfolio is investigated in [28]. A cost minimization for reducing the effect of intermittency in a solar docking charging station with EVs is proposed in [29]. An innovative integrated topology for RES and EVs is proposed and experimentally validated in [30]. A harmonized scheduling of distributed energy assets, optimizing the energy management of a smart home is offered in [31]. The optimization of a smart home prospecting demand response strategies is presented in [32]. A smart charging management for EVs in smart homes is proposed in [33], a control methodology for the EV charging, considering RES and uncertainties as for the energy price, is proposed in [34], and a demand-side energy management including EVs, ESS, RES is presented in [35].

From a global point of view, a complete outline about the status and issues toward the vehicle electrification is offered in [36], whereas an economic investigation of consumers’ lookout for the electric mobility supremacy is presented in [37]. The impact that vehicle electrification can cause in the electrical grid is presented in [38], and a survey concerning the vehicle electrification encompassed in a smart grid background is presented in [39]. On the other hand, as the title indicates, this paper focuses on the challenges and opportunities that arise from vehicle electrification, concretely in terms of the utilization of the on-board and off-board EV battery chargers (EVBCs) for innovative operation modes. Thus, besides the traditional operation modes, grid-to-vehicle (G2V) and vehicle-to-grid (V2G), this paper focuses on the possibility to integrate power quality features in the on-board and off-board EVBCs, as a contribution for the grid-side, as well as on the framework with unified technologies with RES and ESS.

Contextualizing the aforementioned aspects, harmonized for vehicle electrification, the main contributions of this paper encompass proposals in the following areas: (a) New opportunities of operation toward on-board EVBCs in a future perspective of smart homes; (b) new opportunities of operation toward single- and three-phase off-board EVBCs in a future perspective of smart grids; (c) new operation modes of off-board EVBCs considering the perspective of improving power quality aspects for the grid-side; (d) new operation modes for single- and three-phase off-board EVBCs considering a unified integration with RES.

After this brief introduction, Section2introduces the operating principle of an EVBC, highlighting the different configurations of on-board and off-board strands. Section3comprehensively presents the challenges and opportunities that on-board and off-board EVBCs represent for smart grids and smart homes (considering single- and three-phase interfaces). Section4presents three laboratory

(3)

Energies 2019, 12, 118 3 of 20

prototypes of EVBCs encompassing innovative features, as well as a brief experimental validation. Finally, Section5highlights the main conclusions that can be drawn from this paper.

2. EV Battery Chargers: Principle of Operation

This section introduces the principle of operation of EV battery chargers (EVBCs), as well as the future perspectives in terms of on-board and off-board systems, wired and wireless systems, and integrated coordination towards smart grids.

2.1. On-Board and Off-Board Systems

The principle of operation of an on-board and an off-board EVBC, also highlighting its internal constitution, is presented in this section. Internally, an EVBC is composed of power electronics converters and their control systems, responsible for controlling the EV battery charging and, in conjunction with the other elements of the EV, for establishing a communication with the energy management system of the smart grid or smart home with the concrete objective of defining set points of operation. Figure1shows the basic and classical structure of an EVBC, composed by two power converters (a grid-side one interfacing with the electrical grid and a battery-side one interfacing with the EV battery) and by the digital control system common to both power converters. Since the control is done with a closed-loop algorithm, this figure also shows the main control variables that are necessary to acquire, as well as the output control signals for the semiconductors of the power converters.

Energies 2018, 11, x FOR PEER REVIEW 3 of 20

experimental validation. Finally, Section 5 highlights the main conclusions that can be drawn from this paper.

2. EV Battery Chargers: Principle of Operation

This section introduces the principle of operation of EV battery chargers (EVBCs), as well as the future perspectives in terms of on-board and off-board systems, wired and wireless systems, and integrated coordination towards smart grids.

2.1. On-Board and Off-Board Systems

The principle of operation of an on-board and an off-board EVBC, also highlighting its internal constitution, is presented in this section. Internally, an EVBC is composed of power electronics converters and their control systems, responsible for controlling the EV battery charging and, in conjunction with the other elements of the EV, for establishing a communication with the energy management system of the smart grid or smart home with the concrete objective of defining set points of operation. Figure 1 shows the basic and classical structure of an EVBC, composed by two power converters (a grid-side one interfacing with the electrical grid and a battery-side one interfacing with the EV battery) and by the digital control system common to both power converters. Since the control is done with a closed-loop algorithm, this figure also shows the main control variables that are necessary to acquire, as well as the output control signals for the semiconductors of the power converters.

Figure 1. Structure of an electric vehicle battery charger (EVBC) composed of two power converters

(a grid-side one interfacing the electrical grid and a battery-side one interfacing the EV battery) and the digital control system.

This is the customary organization of an EVBC, however, it can be classified according to its arrangement with respect to the EV, i.e., on-board and off-board. An EVBC is classified as on-board when the power electronics required to charge the EV battery are inside the EV, i.e., the converter responsible for controlling the stages of battery charging is inside the EV (usually more than a single controlled stage of voltage and current). Figure 2 shows the interface of an EV with the power grid through an on-board EVBC and an off-board EVBC. The power converters of the on-board EVBC are responsible for the bidirectional power flow between the electrical grid and the EV battery. For the grid-side, the ac-dc converter can be controlled by current or voltage according to the operating mode and for the battery-side, the dc-dc converter can also be controlled by current or voltage according to the intended operating mode for the charging system (c.f. Section 3). As shown in the figure, the operating mode is defined by specific control algorithms, whose management is in accordance with the information from the battery management system (BMS), i.e., the BMS establishes the limits of voltage and current during the charging or discharging processes. On the other hand, an EVBC is classified as off-board when the power electronics required to charge the EV battery are outside the EV, i.e., the converters that are responsible for controlling the battery charging stages (usually a single current controlled stage) are outside the EV. An off-board EVBC is composed by a grid-side converter

PW M PW M Electrical Grid EV battery dc dc dc ac

Figure 1.Structure of an electric vehicle battery charger (EVBC) composed of two power converters (a grid-side one interfacing the electrical grid and a battery-side one interfacing the EV battery) and the digital control system.

This is the customary organization of an EVBC, however, it can be classified according to its arrangement with respect to the EV, i.e., on-board and off-board. An EVBC is classified as on-board when the power electronics required to charge the EV battery are inside the EV, i.e., the converter responsible for controlling the stages of battery charging is inside the EV (usually more than a single controlled stage of voltage and current). Figure2shows the interface of an EV with the power grid through an on-board EVBC and an off-board EVBC. The power converters of the on-board EVBC are responsible for the bidirectional power flow between the electrical grid and the EV battery. For the grid-side, the ac-dc converter can be controlled by current or voltage according to the operating mode and for the battery-side, the dc-dc converter can also be controlled by current or voltage according to the intended operating mode for the charging system (c.f. Section3). As shown in the figure, the operating mode is defined by specific control algorithms, whose management is in accordance with the information from the battery management system (BMS), i.e., the BMS establishes the limits of voltage and current during the charging or discharging processes. On the other hand, an EVBC is classified as off-board when the power electronics required to charge the EV battery are outside the EV, i.e., the converters that are responsible for controlling the battery charging stages (usually a single current controlled stage) are outside the EV. An off-board EVBC is composed by a grid-side converter

(4)

Energies 2019, 12, 118 4 of 20

and by a battery-side converter, both allowing bidirectional power flow between the electrical grid (with current control) and the EV battery (also with current control), i.e., in both cases, it is similar to the operation presented previously for the on-board EVBC. Moreover, for the off-board structure, the control of the operating mode is also defined in accordance with the information provided by the BMS.

Figure 2. Interface of an EV with the power grid through an on-board EVBC and an off-board EVBC. 2.2. Wireless Charging Systems

In the previous section the on-board and off-board EV battery chargers (EVBCs) were introduced. Due to weight and volume restrictions from the EV perspective, normally on-board EVBCs are designed for far lower power ratings than off-board EVBCs. However, it is important to distinguish that, from the electrical grid point of view, in both cases, the EVBCs can have a galvanic isolation or not, either for safety reasons or for convenience, in order to reduce operating voltage levels (i.e., the levels between the grid and the EV battery). In addition to the galvanic isolation that can be implemented for on-board and off-board systems, EVBCs can also be classified as wired or wireless, depending on whether there is a physical link between the electrical grid and the EVBC. Usually, in a wireless system, a part of the power electronics converter is outside the EV (off-board) and the other part is inside the EV (on-board).

Wireless charging systems are becoming popular for different appliances and for EVs; therefore, the main automakers are also developing realistic solutions for their EVs, including a significant range of charging levels. These wireless charging systems, which have been explored by different companies over the last decades, are also seen as a key opportunity to disseminate the electric mobility market, since it is a new exciting experience for the user. Complete overviews about wireless charging technologies for applications in electric mobility are presented in [40–43]. A basic wireless charging system consists in a fixed ground pad that stays below the EV during the charging and a receiving system that stays embedded in the inferior part of the EV. In addition to the need to increase the efficiency of the power transfer between the ground pad and the EV, which will involve the use of innovative technologies of power converters, the full adoption of wireless charging systems will rely on industry standards, universal communication with any EV and the charging pad, and safety issues for human beings and animals.

2.3. EV in Smart Grids: Coordination and Power Quality

As demonstrated in [44] and [45], the EV dissemination signifies a vast contribution for electrical grids, both in terms of future trends and control coordinating strategies. For example, the collaborative operation between EVs and RES is introduced in [46], and the contextualization with smart homes and microgrids is presented in [47] and [48]. Taking into account the EV operation in G2V and V2G modes framed in smart grids, several key points can be addressed. For instance, the impact on distribution systems is analyzed in [49], the coordination with RES scenarios is explored in [50], the contribution to reducing operating costs and to regulating the grid voltage frequency is explored in [51], and the dynamic operation as a function of other appliances is investigated in [52]. Besides the G2V and V2G modes, the EV can also contribute to improving power quality issues. For instance, the EVBC operation as an active filter is introduced in [53] and [54], and the EVBC

Communication Battery Management System (BMS) EV battery Electrical

Grid on-boardEVBC off-boardEVBC ElectricalGrid

Power (slow charging) Power (fast charging)

Figure 2.Interface of an EV with the power grid through an on-board EVBC and an off-board EVBC.

2.2. Wireless Charging Systems

In the previous section the on-board and off-board EV battery chargers (EVBCs) were introduced. Due to weight and volume restrictions from the EV perspective, normally on-board EVBCs are designed for far lower power ratings than off-board EVBCs. However, it is important to distinguish that, from the electrical grid point of view, in both cases, the EVBCs can have a galvanic isolation or not, either for safety reasons or for convenience, in order to reduce operating voltage levels (i.e., the levels between the grid and the EV battery). In addition to the galvanic isolation that can be implemented for on-board and off-board systems, EVBCs can also be classified as wired or wireless, depending on whether there is a physical link between the electrical grid and the EVBC. Usually, in a wireless system, a part of the power electronics converter is outside the EV (off-board) and the other part is inside the EV (on-board). Wireless charging systems are becoming popular for different appliances and for EVs; therefore, the main automakers are also developing realistic solutions for their EVs, including a significant range of charging levels. These wireless charging systems, which have been explored by different companies over the last decades, are also seen as a key opportunity to disseminate the electric mobility market, since it is a new exciting experience for the user. Complete overviews about wireless charging technologies for applications in electric mobility are presented in [40–43]. A basic wireless charging system consists in a fixed ground pad that stays below the EV during the charging and a receiving system that stays embedded in the inferior part of the EV. In addition to the need to increase the efficiency of the power transfer between the ground pad and the EV, which will involve the use of innovative technologies of power converters, the full adoption of wireless charging systems will rely on industry standards, universal communication with any EV and the charging pad, and safety issues for human beings and animals.

2.3. EV in Smart Grids: Coordination and Power Quality

As demonstrated in [44] and [45], the EV dissemination signifies a vast contribution for electrical grids, both in terms of future trends and control coordinating strategies. For example, the collaborative operation between EVs and RES is introduced in [46], and the contextualization with smart homes and microgrids is presented in [47] and [48]. Taking into account the EV operation in G2V and V2G modes framed in smart grids, several key points can be addressed. For instance, the impact on distribution systems is analyzed in [49], the coordination with RES scenarios is explored in [50], the contribution to reducing operating costs and to regulating the grid voltage frequency is explored in [51], and the dynamic operation as a function of other appliances is investigated in [52]. Besides the G2V and V2G modes, the EV can also contribute to improving power quality issues. For instance, the EVBC operation as an active filter is introduced in [53] and [54], and the EVBC contribution for compensating reactive power in the electrical grid is investigated in [55] and [56]. In the context of power quality, an overview

(5)

Energies 2019, 12, 118 5 of 20

about power quality in smart grids is established in [57], a collaborative support between RES and EVs for enhancing power grid support is analyzed in [58], the key aspects of the EVs integration into smart grids are discussed in [59], and innovative operations for the EVs connected into power grids toward mitigating issues of power quality are proposed in [60].

3. Opportunities for Smart Grids

Section2introduced the different structures that can be considered for an EVBC. As the analysis of the structures in terms of power electronics is not the objective of this paper, but the challenges and opportunities of vehicle electrification in smart grids and smart homes, the particular details of the topologies of the converters, either hardware or software, are not presented in this section.

3.1. On-Board EV Battery Charger

This section presents the main operation modes of an on-board EVBC, taking into account its limitations and the opportunities that they can offer for the operation in smart grids and smart homes, concretely, in terms of power controllability and new functionalities obtained for the installation where the EV is plugged-in. As an example case, Figure3illustrates the integration of an EV (including the on-board EVBC) into a smart home. As shown, the EV battery is charged through an on-board EVBC, which is connected to the electrical grid in parallel with the home loads, i.e., when present, the EV is treated as an additional home load. As illustrated, bidirectional communication is considered between the smart home and the electrical grid toward a smart grid perspective in terms of controllability.

contribution for compensating reactive power in the electrical grid is investigated in [55] and [56]. In the context of power quality, an overview about power quality in smart grids is established in [57], a collaborative support between RES and EVs for enhancing power grid support is analyzed in [58], the key aspects of the EVs integration into smart grids are discussed in [59], and innovative operations for the EVs connected into power grids toward mitigating issues of power quality are proposed in [60].

3. Opportunities for Smart Grids

Section 2 introduced the different structures that can be considered for an EVBC. As the analysis of the structures in terms of power electronics is not the objective of this paper, but the challenges and opportunities of vehicle electrification in smart grids and smart homes, the particular details of the topologies of the converters, either hardware or software, are not presented in this section.

3.1. On-Board EV Battery Charger

This section presents the main operation modes of an on-board EVBC, taking into account its limitations and the opportunities that they can offer for the operation in smart grids and smart homes, concretely, in terms of power controllability and new functionalities obtained for the installation where the EV is plugged-in. As an example case, Figure 3 illustrates the integration of an EV (including the on-board EVBC) into a smart home. As shown, the EV battery is charged through an on-board EVBC, which is connected to the electrical grid in parallel with the home loads, i.e., when present, the EV is treated as an additional home load. As illustrated, bidirectional communication is considered between the smart home and the electrical grid toward a smart grid perspective in terms of controllability.

Figure 3. Illustration of the integration of an on-board EVBC into a smart home.

3.1.1. Grid-to-Vehicle (G2V)

bat

v

hl

Figure 3.Illustration of the integration of an on-board EVBC into a smart home.

3.1.1. Grid-to-Vehicle (G2V)

The G2V operation mode is exclusively concerned with the EV battery charging directly from the grid, and it is usually the only mode of operation available on EVs. As exemplified in Figure4, the on-board EVBC is connected to the electrical grid through a smart home with unidirectional power flow and with bidirectional communication between the smart home, the electrical grid and the on-board EVBC. With this operation mode, the value of the EVBC grid-side current (iev) does not take into account the other loads connected in the same electrical installation (e.g., in the case of a home, the total current is limited by the main circuit breaker, which may be triggered if the limit is surpassed). The principle of operation representative of the G2V mode in a smart home is presented in Figure5, where the grid voltage (vg), the grid current (ig), the home loads current (ihl) and the EVBC grid-side current (iev) are represented. In order to avoid deteriorating the power quality indices in the electrical

(6)

Energies 2019, 12, 118 6 of 20

grid, the EVBC current is sinusoidal and in phase with the grid voltage. As shown, realistic conditions are considered in terms of distorted grid voltage (vg) and home loads current (ihl).

electrical grid, the EVBC current is sinusoidal and in phase with the grid voltage. As shown, realistic conditions are considered in terms of distorted grid voltage (vg) and home loads current (ihl).

Figure 4. On-board EVBC: Grid-to-vehicle (G2V) operation mode.

Figure 5. Principle of operation representative of the G2V mode.

Similar to the (basic) G2V mode, the controlled G2V mode refers to the EV battery charging directly from the grid, but with adjustment of the operating power value according to the other connected loads [52]. Besides, with this operation mode, for example, the EVBC operating power may be adjusted according to the power injected by RES, aiming to balance the power production and consumption from the smart home perspective, and without harming the power quality on the grid-side (e.g., frequency and amplitude deviations on the grid voltage). In order to implement this operation mode, it is necessary to establish a communication between the EVBC and the grid (or the home energy management system, i.e., when considering the EV integration into a smart home). The principle of operation representative of the controlled G2V mode is exemplified in Figure 6. Similarly to the aforementioned G2V mode, the EVBC operates with a sinusoidal grid-side current; however, its amplitude is adjusted in real-time according to the other loads of the home. In the transition from case #1 to case #2, a home load was turned-off (the current consumption, ihl, decreases), so the EVBC

increases its operating power (increases the current consumption, iev). Nevertheless, the maximum

operating power of the EVBC, which is internally controlled, cannot be exceeded in any circumstance. Applying this control strategy to the EV battery charging, the maximum operating power of the smart home is never exceeded, maintaining the same value. This can be observable in the amplitude of the grid current (ig).

Figure 6. Principle of operation representative of the controlled G2V mode.

_ev #1 #2

Figure 5.Principle of operation representative of the G2V mode.

Similar to the (basic) G2V mode, the controlled G2V mode refers to the EV battery charging directly from the grid, but with adjustment of the operating power value according to the other connected loads [52]. Besides, with this operation mode, for example, the EVBC operating power may be adjusted according to the power injected by RES, aiming to balance the power production and consumption from the smart home perspective, and without harming the power quality on the grid-side (e.g., frequency and amplitude deviations on the grid voltage). In order to implement this operation mode, it is necessary to establish a communication between the EVBC and the grid (or the home energy management system, i.e., when considering the EV integration into a smart home). The principle of operation representative of the controlled G2V mode is exemplified in Figure6. Similarly to the aforementioned G2V mode, the EVBC operates with a sinusoidal grid-side current; however, its amplitude is adjusted in real-time according to the other loads of the home. In the transition from case #1 to case #2, a home load was turned-off (the current consumption, ihl, decreases), so the EVBC increases its operating power (increases the current consumption, iev). Nevertheless, the maximum operating power of the EVBC, which is internally controlled, cannot be exceeded in any circumstance. Applying this control strategy to the EV battery charging, the maximum operating power of the smart home is never exceeded, maintaining the same value. This can be observable in the amplitude of the grid current (ig).

3.1.2. Vehicle-to-Grid (V2G)

The V2G operation mode refers to the return of part of the energy stored in the EV battery to the grid conferring to the convenience of the grid management system and the EV user, representing a benefit for the electrical grid, because it allows using the EV as an ESS for supporting the grid stability. Contrary to the G2V, in this operation mode, the grid-side and the battery-side converters must be used in bidirectional mode, representing a perspective for the EVBCs of the future EVs. Moreover, this mode requires communication with a grid aggregator, in order to define in which schedules the EVBC operates in this mode, as well as the amount of power that is necessary to return to the grid. This operation mode, illustrated in Figure7, is controlled according to the power injected into the grid, but it can also be controlled based on the loads connected in the same electrical installation. Figure8 presents some results illustrating the V2G mode. Initially, in case #1, the EVBC is operating in V2G

(7)

Energies 2019, 12, 118 7 of 20

mode by injecting power into the grid without any control over the other loads, and then, in case #2, the EVBC injects power into the grid as a function of the other loads. In this specific case, a load was turned off; therefore, the power injected increases proportionally. As can be observed, in both cases, the EVBC grid-side current is in phase opposition with the voltage, meaning that power is injected into the grid.

_ev #1 #2

Figure 6.Principle of operation representative of the controlled G2V mode.

The V2G operation mode refers to the return of part of the energy stored in the EV battery to the grid conferring to the convenience of the grid management system and the EV user, representing a benefit for the electrical grid, because it allows using the EV as an ESS for supporting the grid stability. Contrary to the G2V, in this operation mode, the grid-side and the battery-side converters must be used in bidirectional mode, representing a perspective for the EVBCs of the future EVs. Moreover, this mode requires communication with a grid aggregator, in order to define in which schedules the EVBC operates in this mode, as well as the amount of power that is necessary to return to the grid. This operation mode, illustrated in Figure 7, is controlled according to the power injected into the grid, but it can also be controlled based on the loads connected in the same electrical installation. Figure 8 presents some results illustrating the V2G mode. Initially, in case #1, the EVBC is operating in V2G mode by injecting power into the grid without any control over the other loads, and then, in case #2, the EVBC injects power into the grid as a function of the other loads. In this specific case, a load was turned off; therefore, the power injected increases proportionally. As can be observed, in both cases, the EVBC grid-side current is in phase opposition with the voltage, meaning that power is injected into the grid.

Figure 7. On-board EVBC: vehicle-to-grid (V2G) operation mode.

Figure 8. The principle of operation representative of the V2G mode.

3.1.3. Vehicle-to-Load (V2L)–Voltage Source

In the previously presented operation modes, the EVBC is controlled in order to absorb or inject power into the grid, where the grid-side converter operates with a current feedback control, i.e., the voltage is imposed by the grid and the EVBC defines the current waveform. In the operation mode as a voltage source, the EVBC operates independently from the grid, i.e., it can be used as a voltage source to power loads according to the user's convenience. The principle of operation representative of the vehicle-to-load (V2L) mode, i.e., as a voltage source, is presented in Figure 9. This operation mode is useful, for example, in remote locations where a voltage source is only necessary for short periods. It may also be useful in campsites, or in extreme situations of catastrophic events where the grid may be unavailable. Thus, in this operation mode, the grid-side converter operates with a voltage feedback control, i.e., the voltage is imposed by the EVBC and the current waveform is defined by the linear or nonlinear loads connected to the EVBC. As operation mode uses the energy from the EV battery, the EV owner is responsible for the management of the battery state-of-charge, e.g., regarding the minimum acceptable state-of-charge for the next travel. Internally, the EV battery is protected by

Electrical Grid Smart Home on-board EVBC 0 V 200 V -20 A 0 A 20 A 0.05 s 0.06 s 0.07 s 0.08 s 0.09 s 0.10 s 0.11 s 0.12 s 0.13 s 0.14 s 0.15 s 0 V 200 V -20 A 0 A 20 A -400 V 0 V 400 V -40 A 0 A 40 A

v

g

i

_hl

i

_g

i

ev #1 #2

Figure 7.On-board EVBC: vehicle-to-grid (V2G) operation mode.

The V2G operation mode refers to the return of part of the energy stored in the EV battery to the grid conferring to the convenience of the grid management system and the EV user, representing a benefit for the electrical grid, because it allows using the EV as an ESS for supporting the grid stability. Contrary to the G2V, in this operation mode, the grid-side and the battery-side converters must be used in bidirectional mode, representing a perspective for the EVBCs of the future EVs. Moreover, this mode requires communication with a grid aggregator, in order to define in which schedules the EVBC operates in this mode, as well as the amount of power that is necessary to return to the grid. This operation mode, illustrated in Figure 7, is controlled according to the power injected into the grid, but it can also be controlled based on the loads connected in the same electrical installation. Figure 8 presents some results illustrating the V2G mode. Initially, in case #1, the EVBC is operating in V2G mode by injecting power into the grid without any control over the other loads, and then, in case #2, the EVBC injects power into the grid as a function of the other loads. In this specific case, a load was turned off; therefore, the power injected increases proportionally. As can be observed, in both cases, the EVBC grid-side current is in phase opposition with the voltage, meaning that power is injected into the grid.

Figure 7. On-board EVBC: vehicle-to-grid (V2G) operation mode.

Figure 8. The principle of operation representative of the V2G mode.

In the previously presented operation modes, the EVBC is controlled in order to absorb or inject power into the grid, where the grid-side converter operates with a current feedback control, i.e., the voltage is imposed by the grid and the EVBC defines the current waveform. In the operation mode as a voltage source, the EVBC operates independently from the grid, i.e., it can be used as a voltage source to power loads according to the user's convenience. The principle of operation representative of the vehicle-to-load (V2L) mode, i.e., as a voltage source, is presented in Figure 9. This operation mode is useful, for example, in remote locations where a voltage source is only necessary for short periods. It may also be useful in campsites, or in extreme situations of catastrophic events where the grid may be unavailable. Thus, in this operation mode, the grid-side converter operates with a voltage feedback control, i.e., the voltage is imposed by the EVBC and the current waveform is defined by the linear or nonlinear loads connected to the EVBC. As operation mode uses the energy from the EV battery, the EV owner is responsible for the management of the battery state-of-charge, e.g., regarding the minimum acceptable state-of-charge for the next travel. Internally, the EV battery is protected by

Electrical Grid Smart Home on-board EVBC 0 V 200 V -20 A 0 A 20 A 0.05 s 0.06 s 0.07 s 0.08 s 0.09 s 0.10 s 0.11 s 0.12 s 0.13 s 0.14 s 0.15 s 0 V 200 V -20 A 0 A 20 A -400 V 0 V 400 V -40 A 0 A 40 A

v

g

i

_hl

i

_g

i

_ev #1 #2

Figure 8.The principle of operation representative of the V2G mode.

In the previously presented operation modes, the EVBC is controlled in order to absorb or inject power into the grid, where the grid-side converter operates with a current feedback control, i.e., the voltage is imposed by the grid and the EVBC defines the current waveform. In the operation mode as a voltage source, the EVBC operates independently from the grid, i.e., it can be used as a voltage source to power loads according to the user’s convenience. The principle of operation representative of the vehicle-to-load (V2L) mode, i.e., as a voltage source, is presented in Figure9. This operation mode is useful, for example, in remote locations where a voltage source is only necessary for short periods. It may also be useful in campsites, or in extreme situations of catastrophic events where the grid may be unavailable. Thus, in this operation mode, the grid-side converter operates with a voltage feedback control, i.e., the voltage is imposed by the EVBC and the current waveform is defined by the linear or nonlinear loads connected to the EVBC. As operation mode uses the energy from the EV

(8)

Energies 2019, 12, 118 8 of 20

battery, the EV owner is responsible for the management of the battery state-of-charge, e.g., regarding the minimum acceptable state-of-charge for the next travel. Internally, the EV battery is protected by the BMS. Since this operation mode can be used in multiple locations and for various purposes (e.g., smart homes, remote locations, or in islanding mode), it represents a new contribution for the future smart grids. It is meaningful to note that Nissan already has a system entitle “LEAF-to-Home”, where the “EV Power Station” interfaces an EV and a house [61]. However, the key drawback of the Nissan system is that it can only be used where it is installed, i.e., it cannot be used generically with the EV in any place other than the home.

In Figure10the operating principle of this mode is presented, where igrepresents the home current, ievrepresents the EVBC grid-side current, and ihlthe loads current. As can be seen, when the EVBC is functioning as a voltage source, the EVBC current is the same as the load current, and the voltage applied to the loads is the voltage produced by the EVBC, whose value is equal to the nominal value of the grid voltage. This figure is divided into three distinct cases. In case #1, the EVBC is not operating in any mode. In case #2, the EVBC is not connected to the grid and starts to produce a sinusoidal voltage, but no load has yet been connected to the EVBC. In case #3, the EVBC is producing a sinusoidal voltage and a load is connected to the EVBC. In this case, since a nonlinear load was connected, it results in a consumed current with a high harmonic content.

the BMS. Since this operation mode can be used in multiple locations and for various purposes (e.g., smart homes, remote locations, or in islanding mode), it represents a new contribution for the future smart grids. It is meaningful to note that Nissan already has a system entitle “LEAF-to-Home”, where the “EV Power Station” interfaces an EV and a house [61]. However, the key drawback of the Nissan system is that it can only be used where it is installed, i.e., it cannot be used generically with the EV in any place other than the home.

In Figure 10 the operating principle of this mode is presented, where ig represents the home

current, iev represents the EVBC grid-side current, and ihl the loads current. As can be seen, when the

EVBC is functioning as a voltage source, the EVBC current is the same as the load current, and the voltage applied to the loads is the voltage produced by the EVBC, whose value is equal to the nominal value of the grid voltage. This figure is divided into three distinct cases. In case #1, the EVBC is not operating in any mode. In case #2, the EVBC is not connected to the grid and starts to produce a sinusoidal voltage, but no load has yet been connected to the EVBC. In case #3, the EVBC is producing a sinusoidal voltage and a load is connected to the EVBC. In this case, since a nonlinear load was connected, it results in a consumed current with a high harmonic content.

Figure 9. On-board EVBC: Vehicle-to-load (V2L) operation mode (as a voltage source).

Figure 10. The principle of operation representative of the V2L mode (as a voltage source).

3.1.4. Vehicle-to-Home (V2H)–Uninterruptible Power Supply

In addition to the operation mode presented earlier, the EVBC can also operate as a voltage source, but with the characteristics of an off-line uninterruptible power supply (UPS). This mode represents a new opportunity for smart homes because, in the event of a power failure, the EVBC can operate almost instantly as a voltage source for the smart home. In this operation mode, communication is required between the EVBC and the smart home, in order to identify a power outage and even to some selected priority loads. The principle of operation representative of the vehicle-to-home (V2H) mode as a UPS is presented in Figure 11, which clearly identifies that the EVBC operates in unidirectional mode and disconnected from the electrical grid.

Like the previously presented operation mode, the grid-side converter of the EVBC operates with a voltage control feedback and the current is defined by the loads. However, unlike the previous mode, it is necessary to measure the grid voltage in order to detect when a voltage failure happens. Whenever this occurs, a control signal is sent to the general circuit breaker, to isolate the loads from the grid, and, almost instantaneously, the EVBC starts its operation as a voltage source. Later, when the grid voltage is restored, the EVBC recognizes this situation. Then, after some cycles of the grid voltage, it begins a synchronization with the phase of the voltage and, as soon as the control system is synchronized, it makes the transition to the normal mode, i.e., the loads are fed again by the grid. After this process, the EVBC may either go to an idle state or start another operation mode, such as G2V or V2G. Figure 12 illustrates the operating principle of this mode, showing the grid voltage (vg),

Electrical Grid Smart Home on-board EVBC Electrical Loads 0 V 200 V -20 A 0 A 20 A 0.05 s 0.06 s 0.07 s 0.08 s 0.09 s 0.10 s 0.11 s 0.12 s 0.13 s 0.14 s 0.15 s -400 V 0 V 400 V -20 A 0 A 20 A

v

_ev

i

_hl

i

_ev #2 #3 #1

Figure 9.On-board EVBC: Vehicle-to-load (V2L) operation mode (as a voltage source).